The present invention relates to a method of purifying a siloxane compound. More particularly, the present invention relates to a method of making a purified siloxane feedstock for use in the manufacturing of silica glass.
There are various processes that involve the production of silica glass from reactants containing silicon. Such processes include a feedstock containing silicon and a means for transporting the feedstock to a conversion site where the feedstock is converted into silica. An apparatus used in such a process usually consists of a liquid feedstock reservoir, a means of converting said liquid feedstock into a vapor and delivering the vaporized feedstock to a burner where the feedstock is converted by flame hydrolysis into silica soot. Silica soot produced by such a burner is then collected on a deposition surface. Such collected silica may be simultaneously or subsequently heat treated to form a fused non-porous transparent high purity silica glass article, particularly well suited for optical applications.
Historically, silicon tetrachloride has been used as the main silicon containing feedstock that is converted to silica. This use presents disadvantages in that environmentally unsafe HCl is produced as a byproduct. As an alternative to using silicon tetrachloride as a feedstock, silica may be formed from halide-free, silicon-containing feedstocks such as siloxanes. U.S. Pat. No. 5,043,002 to Dobbins et al., the disclosure of which is hereby incorporated by reference, discloses the use of siloxane feedstocks. U.S. Pat. No. 5,043,002 to Dobbins et al. discloses the benefits of using polyalkylsiloxanes such as polymethylsiloxanes, and in particular polymethylcyclosiloxanes with octamethylcyclotetrasiloxane as the preferred silicon-containing feedstock, in the manufacturing of silica glass.
It has been found that the use of such siloxane feedstocks, and in particular octamethylcyclotetrasiloxane, in the manufacture of silica glass poses problems caused by the siloxanes"" tendency to polymerize and form higher molecular weight siloxanes which form polymeric deposits such as gels under the conditions and environments of the silica glass manufacturing process. Siloxane feedstocks form troublesome polymeric compounds which deposit in the vaporization, delivery, and conversion equipment used in the silica glass manufacturing process. This polymerizing of the siloxane feedstock causes manufacturing difficulties due to the clogging and restriction of the feedstock flow during storage, vaporization, delivery and conversion. These deposits create great difficulty in controlling, monitoring, and metering the amount and rate at which the siloxane feedstock is utilized in the silica glass manufacturing process and also gel globs which spew out of the burner may produce inhomogeneities which would produce defects in the silica glass.
U.S. patent application Ser. No. 08/574,961 by Henderson and Powers, the disclosure of which is hereby incorporated by reference, discloses a method of purifying polyalkylsiloxanes by removing impurities having boiling points greater than 250xc2x0 C. by distillation in order to avoid the problem of polymeric deposits and gel formation when using the purified polyalkylsiloxane as a siloxane feedstock in the silica manufacturing process.
Accordingly, the present invention is directed to a method of making a high purity polyalkylsiloxane that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the processes, apparatuses, and compositions particularly pointed out in the description and claims.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the invention includes the method of making a high purity nonpolar polyalkylsiloxane. This method includes the step of providing a refined polyalkylsiloxane starting material which has polar impurities and is made up of greater than 95% by weight of the nonpolar polyalkylsiloxane which is the polyalkylsiloxane target product of the invention. The method further includes the step of extracting the polar impurities from the starting material to produce a high purity nonpolar polyalkylsiloxane product.
In another aspect, the invention includes a method of making a polymeric deposit inhibited siloxane feedstock which is resistant to polymer formation and not prone to form polymeric deposits. This method includes the steps of providing a starting siloxane feedstock which has polymeric deposit forming impurities and then solid phase extracting the polymeric deposit forming impurities from the starting siloxane feedstock in order to produce a polymeric deposit inhibited siloxane feedstock.
It is to be understood that both the foregoing general description, and the following detailed description are exemplary and explanatory and are intended to produce further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments, and aspects of the invention and together with the description serve to explain the principle of the invention.